You think it's worth mounting on a humvee over a MG for city warfare?
No, if it's only advantage over a regular shotgun is that it can fire explosive rounds there is already the MK19 to do that and over a greater range with more explosive payload. And the MK19 can fire canister rounds with again a greater payload than a twelve gauge.
As for its bolteryness, sure in its implementation it's a special upgrade weapon, but in what it actually *is* and what it *does* it is basically a boltgun (12 gauge is .72 caliber, bolters are .75, pretty close).
It is, for all intents and purposes, a proto-bolter.